The invention relates to a drive train control arrangement that comprises at least one main drive train, a first drive axle driven permanently by the main drive train, at least one auxiliary drive train and a second drive axle coupleable to the main drive train by the auxiliary drive train, including at least one first clutch unit and at least a second clutch unit connected in the auxiliary drive train downstream of the first clutch unit for connecting the second drive axle to the main drive train, with a control unit, which controls the actuation of the clutches and as a result, the power transferred to the second drive axle.
German Patent Document DE 10 2006 045 007 discloses a motor vehicle drive train comprising a main drive train, a first drive axle permanently driven connected to the main drive train, an auxiliary drive train and a second drive axle coupleable to the main drive train by way of the auxiliary drive train, wherein the auxiliary drive train has a first clutch unit for connecting the second drive axle to the main drive train and a second clutch unit connected downstream in the power train of the first clutch unit.
It is the object of the present invention to provide a drive train control arrangement for a motor vehicle drive train including a main drive train to a first axle and a second drive train to a second axle, automatically actuated for connection of the second drive axle when necessary.
In a drive train control arrangement for a motor vehicle drive train, which comprises at least one main drive train, a first drive axle driven permanently by means of the main drive train, at least one auxiliary drive train and a second drive axle coupleable to the main drive train via the auxiliary drive train, which has a first clutch unit and at least a second clutch unit connected downstream in the power train of the first clutch unit for connecting the second drive axle to the main drive train, and a control unit, which operates in at least one of the following operating modes                closes the at least one second clutch unit of the auxiliary drive train to shift into a standby operating mode when a defined operating condition is present, and        with the standby operating mode activated, closes the first clutch unit of the auxiliary drive train to connect the coupleable second drive axle depending on at least one parameter and establishes herewith a 6×4 operating mode or, respectively, opens the second clutch unit to shift to the normal operating mode, in which the auxiliary drive train is also disconnected.        
The invention is based on a drive train control device for a motor vehicle drive train, which comprises at least one main drive train, a first drive axle driven permanently by means of the main drive train, at least one auxiliary drive train and a second drive axle coupleable to the main drive train by means of the auxiliary drive train, wherein the auxiliary drive train has at least a first clutch unit and at least a second clutch unit connected downstream in the power train of the first clutch unit, with a control unit, which at least shifts a standby operating mode in an operating state when a defined operating condition is present by closing the at least one second clutch unit and, starting from the standby operating mode, connects the coupleable second drive axle depending on at least one parameter by closing the at least one first clutch unit of the auxiliary drive train. A connection of the coupleable second drive axle can thereby be established rapidly. The connection of the second drive axle can be rapidly and better established in response to a need of the second drive axle by a rapid coupling. The connection of the second drive axle can advantageously automatically be performed and controlled by the control unit. An “operating condition” is especially meant to be a condition that has to fulfill a value of at least one parameter to activate something. A “parameter” is a measurable value, which describes an operating state of the motor vehicle drive train. A “standby operating mode” is an operating mode, in which the second clutch unit is closed and the first clutch unit is opened. In the standby operating mode, the motor vehicle drive train is prepared for a connection of the coupleable second drive axle. A “6×4 operating mode” is an operating mode, in which the first and the second clutch units are closed, so the coupleable second drive axle is coupled to the permanently driven first drive axle. “Prepared” means in this case that the second drive axle can be connected by only closing a single clutch unit when necessary.
Furthermore, in the present invention the control unit, starting from the standby operating mode, shifts into a normal operating mode when a further operating condition is present, by opening the at least one second clutch unit. By providing an alternative for the connection of the second drive axle, an adaptation ability of the drive train control device can be increased. A “normal operating mode” is especially meant to be an operating mode in which the first clutch unit and the second clutch unit are opened. In the normal operating mode, the motor vehicle drive train can advantageously be operated in a low loss mode compared to the standby operating mode. A low loss mode means that only the main drive train transfers a torque. In the normal operating mode, a part of the auxiliary drive train is decoupled from a rotary movement. The auxiliary drive train is divided operatively in the normal operating mode, wherein an auxiliary drive train branching off the main drive train, which drives the first drive axle, to drive the second drive axle is disconnected.
Additionally, in the present invention the control unit, starting from the normal operating mode, synchronizes the at least one second clutch unit by intermittently closing the at least one first clutch unit for shifting to the standby operating mode. A safe and comfortable shifting from the normal operating mode to the standby operating mode can be achieved thereby. “Intermittently closing” is especially meant to be that the first clutch unit is only closed for synchronizing the at least one second clutch unit. The duration of the closure is especially dependent on a speed difference across the at least one second clutch unit. The closing state is thereby maintained until either the differential speed lies within a defined differential speed band, or a maximum time span deposited in the control unit has lapsed. The duration thereby varies in a region between zero and the maximum duration. The maximum duration is advantageously between 1 and 10 seconds and particularly advantageously between 1 and 5 seconds.
In an advantageous arrangement, the control unit prevents a shifting of the clutch units during a defined time span, whereby a frequent change between the operating modes can be prevented. A “defined time span” is intended to be a time span, whose value is stored in the control unit. The control unit can thereby be provided to increase the value depending on a traction parameter. The value for the defined time span is preferably between 5 and 60 seconds, advantageously between 5 and 30 seconds, and especially advantageously between 5 and 15 seconds. A “traction parameter” means a parameter which describes how much of the torque present at the drive wheels can be transferred to a road. A surface of a road, as for example a dry (normal) road, can be recognized via the traction parameter.
It is advantageous if the control unit determines the operating condition for shifting the standby operating mode and the operating condition for shifting to the normal operating mode depending on the same parameter. The operating conditions can thereby be defined in a simple manner.
It is further advantageous if the motor vehicle drive train has at least one unpowered axle and the control unit determines the parameter by means of the unpowered axle. An advantageous parameter can thereby be provided in a simple manner. An “unpowered axle” is especially meant to be a permanently unpowered axle. The permanently unpowered axle is advantageously permanently decoupled from the main drive train and the auxiliary drive train, that is, a coupling to the main drive train or to the auxiliary drive train is not possible.
Additionally, in the present invention the operating condition for shifting to the standby operating mode and the operating condition for shifting to the normal operating mode are defined by two different speed limit values. Back and forth shifting from the standby operating mode to the normal operating mode and vice versa which is too frequent, can thereby be prevented in an advantageous manner.
In an advantageous arrangement the 6×4 operating mode is established when the vehicle speed is below a third speed limit value, so it is established at launch of the vehicle and at very low vehicle speeds. The control unit can additionally check whether the engine torque is above an engine torque threshold. Drive tire wear is an exponential function of the longitudinal force on the tire. In 6×4 operating mode there is an even distribution of the torque between the first and the second rear drive axle. Establishing the 6×4 operating mode in the named situations produces even tire wear and helps to improve tire life on the first rear drive axle.
It is especially advantageous if the control and/or regulation unit adapts at least one of the conditions for a change of the operation mode to the traction parameter in at least one operating state. The named conditions can thereby be adapted to an environment and to a road surface.
In an advantageous arrangement the control unit monitors an oil temperature of the axle drive of the second drive axle and closes the first clutch unit when the oil temperature is below a temperature threshold. During extended duration of operation in the normal mode (axle drive (differential) of the second drive axle stands still) and during freezing ambient temperatures, it is possible for a second axle sump oil to transform into a semi-solid state. Subsequent engagement of the second clutches during transition to the standby operation mode might cause a torque spike in the axle. By closing the first clutch unit periodically a drive gearwheel of the second axle drive spins and heats up the sump oil.
In an advantageous arrangement the control unit engages the 6×4 operating mode when engine compression brakes are activated. Tire wear is an exponential function of the longitudinal force on the tire. Engaging the 6×4 operating mode provides even distribution of the negative torque between the first and the second axle drive to produce even tire wear. This helps to improve tire life on the first axle drive.
In an advantageous arrangement, the control unit connects a drive shaft of the main drive train to a drive shaft of the auxiliary drive train in at least one operating state by means of the at least a first clutch unit. The connection of the second drive axle can thereby be realized in a simple manner.
Moreover, in the present invention the auxiliary drive train has at least one axle drive (differential) and the second clutch unit at least two clutches arranged in the power train after the axle drive, and the control and/or regulation unit simultaneously opens the two clutches of the second clutch unit for shifting the normal operating mode. The decoupling of the drive wheels of the second drive axle can thereby be realized in a simple manner by the auxiliary drive train.
The invention will become more readily apparent from the following description of a particular embodiment thereof on the basis of the accompanying drawings.